1. Field of the invention
The invention relates to processing units for cleaning small mechanical parts or assemblies and subassemblies of such parts.
2. Description of Prior Art
Such units are well-known. They all include a cleaning unit formed by a tank in which the parts to be cleaned are subjected to the action of the selected solvent and which is topped by a cooling zone surrounded by water-circulating coils with a peripheral gutter at its base for collecting the condensate.
The cleaning is carried out in three operations: liquid phase washing, rinsing by trickling in the vapor phase and drying by condensation (the latter operation being carried out in the cooling zone).
The parts to be cleaned are arranged in a movable basket guided vertically between the bottom of the tank and an upper point situated abov the cooling zone.
The cleaning agents used are volatile solvents, principally chlorinated solvents (chlorotene, trichloroethylene, etc.) or fluorinated solvents of the fluorocarbon type such as, for example, that known commercially as "Freon" or its azeotropic derivatives. The latter type of solvent is generally preferred when possible since it is practially inert even with respect to synthetic materials whose presence is more and more frequent in mechanical assemblies or sub-assemblies.
For the liquid phase cleaning operation, the solvent is generally in circulation and/or constantly stirred. From this point of view it is possible to distinguish two types of cleaning units.
In the first type, the solvent is projected by nozzles towards the basket containing the articles to be cleaned in the form of horizontal jets distributed over the whole height of the basket, the basket and the nozzles being in relative rotation. The solvent is removed from the bottom of the tank to the reservoir and passed through a filtering device. Thus, the tank is completely empty after liquid phase cleaning by sprinkling and the vapor phase rinsing can be effected without moving the basket. The cooling zone is arranged immediately above the tank and by raising the basket again into this zone, drying by evaporation is effected.
In a second type, the cleaning is effected by immersion. The tank is constantly filled with liquid solvent kept in constant circulation by a pump arranged on a pipe connecting a filling orifice (situated generally at mid-height of the tank), on the one hand, and two removal orifices, on the other hand, one for overflow at the top part and the other part for draining at the bottom part. Of course, a filtering device is placed in said the pipe. In addition, the solvent may be subjected to stirring means arranged at the bottom of the tank, which means can be ultrasonic (magnetostriction transducers or ferroelectric ceramic), mechanical (blade stirrer) or dynamic (pulsed air). In this case, the vapor phase rinsing is effected above the level of the liquid after having re-raised the basket into an intermediate zone situatued between the tank proper and the cooling zone.
In all cases, the passage into vapor phase is intended above all to occur on rinsing by trickling but also serves for reheating the cleaned parts in order to facilitate the speed of evaporation when they pass into a cold zone and thus to obtain better drying. In spite of this, mostly, solvent droplets are retained in certain hollows of the parts to be processed. This solvent retention presents a triple drawback. On the one hand, the rinsing is incomplete at the level of these hollows and impurities can persist there which can be serious where precision parts are concerned. On the other hand, it involves an increase in the consumption of solvent which is still an expensive material. Finally, reasons of hygiene and safety require the removal as completely as possible of the solvent.